Migrate enabled state of plugins into specialized Flash and PDF prefs.

For PDF and Flash the enabled state is defined by the
ManagedDefaultPluginsSetting and PluginsAlwaysOpenPdfExternally prefs.
All other plugins should never be disabled in the first place.

BUG=662002
TEST=manual

Review-Url: https://codereview.chromium.org/2474713003
Cr-Commit-Position: refs/heads/master@{#432477}
5 files changed